A 58-year-old previously healthy man presented with abrupt-onset, severe headache followed by rapid neurological deterioration and decreased consciousness (GCS 8/10). Given the acute presentation, toxic ingestion was initially suspected, and he was closely monitored for systemic complications. Shortly after admission, he developed cardiopulmonary arrest, achieving return of spontaneous circulation after advanced resuscitation. He was subsequently intubated and transferred to the intensive care unit with vasopressor support. 

Initial brain CT revealed no acute abnormalities. Abdominal CT demonstrated gastric pneumatosis and portal venous gas, supporting the suspicion of systemic gas embolism. Eight hours after the onset of neurological deterioration, brain MRI showed hyperintense lesions on FLAIR/T2 with diffusion restriction on DWI, consistent with acute to subacute ischemia in the right frontal and parietal territories.

Despite mechanical ventilation and comprehensive critical care, the patient remained comatose and hypoxemic. Further history obtained from the patient’s family later confirmed the accidental ingestion of approximately 100 mL of 30% hydrogen peroxide, less than one hour before symptom onset. Despite maximal supportive therapy, including antibiotics for ventilator-associated pneumonia, no neurological improvement was observed after 20 days of hospitalization.